The present invention generally relates to a method and apparatus for thin film deposition, and more particularly to the controlled deposition of thin films onto a predetermined area of a moving substrate.
Thin film deposition techniques of the prior art include explosively vaporizing an electrical conductor and causing the resulting vapor to condense in a vacuum or in a non-contaminating atmosphere onto a suitable substrate. In non-directed vapor deposition, mass motion is thermally initiated by the radial vapor dispersion accompanying the explosive vaporization of a wire conductor. In directed vapor deposition, vapor is directed towards a substrate by the interaction with electrostatic and electromagnetic fields providing that the vapor particles are charged or of a magnetic nature. In either instance, vapor formation is not suppressed during heating and the distribution in droplet size and temperature depends upon the uniform heating of the conductor.
The prior art is replete with charged particle accelerating devices wherein a current conducting substance, situated between a pair of current conducting rail electrodes, is accelerated by the force resulting from the interaction between the magnetic field between the rail electrodes and the moving charge particles in the conducting substance. Any conducting substance may be accelerated in a linear electric motor of this nature and it is well known to form a current conducting plasma between two rail electrodes by discharging a storage capacitor to explosively vaporize an electrical conductor. The common configuration of a plasma accelerator is such that a magnetic field is built up behind the plasma that is perpendicular to the current in the plasma so that the resultant mutually perpendicular force on the plasma accelerates it down the electrodes. Current discharge is commonly of an under-damped RLC type and is continuously applied to the rail electrodes with full current oscillation. In addition to rail-type plasma guns, Kolb tubes are known utilizing a backstrap electrode having current flow anti-parallel to the current flow in the plasma. Because the direction of force does not change, rail-type guns and Kolb tubes can be operated cyclically to accelerate a neutral plasma to relatively high speeds. Current flow is terminated when the plasma leaves the electrodes, acting as its own switch.
Both rail-type and Kolb tube plasma accelerators can be operationally efficient, but possess certain disadvantages when applied to thin film deposition techniques due to non-uniform mass acceleration. In such systems, when the electrical conductor is explosively vaporized, the heated material expands in all directions and is uncontained in the radial direction away from the wire axis. During initial current rise, the thermal pressure of the plasma exceeds the self-induced magnetic "pinch" pressure resulting from the passage of current through the plasma, and the metal wire propellants explode with a high radial velocity superposed on the directed velocity causing dispersion of the plasma with resultant broad velocity distribution. Further, the electrical resistivity of a plasma has a negative slope with increasing temperature, causing the plasma current to tend to collapse into an arc. Therefore, the plasma is driven toward non-uniform current conduction as it is heated during acceleration adding to non-uniform velocity and mass distributions. The large resultant non-uniformities make it difficult to deposit the vapor on a moving substrate without smearing. Rail inductance is maximized compared to the external discharge circuit inductance to reduce the duration of energy transfer and increase plasma acceleration efficiency; and as a result, the discharge circuit parameters, such as frequency, are continually changing and control over the current magnitude and time variation is difficult to achieve.